Electron beam addressable archival memory

ABSTRACT

An electron beam addressable memory is disclosed wherein information is stored as deformation and phase changes in a noncrystalline thin film. The thin film comprises a material having metastable structural characteristics. Information is stored by locally heating the film to change it from one state to another, thereby deforming it. Readout is accomplished by scanning the film with an electron beam and determining the stored information by variations in secondary electron emission yield due to the deformations and phase changes in the film.

Unite States Patet Chen et al.

1111 3,750,117 1451 July 31,1973

[54] ELECTRON BEAM ADDRESSABLE ARCHIVAL MEMORY inventors: Arthur C. M.Chen; Jish-Min Wang,

both of Schenectady, NY.

General Electric Company, Schenectady, NY.

Filed: Sept. 30, 1971 Appl. No.: 185,125

Assignee:

11.8. Cl. 340/173 LS, 346/74 EB Int. Cl Gllc 11/42 Field of Search340/173 R, 173 LS,

340/173 CR; 346/74 EB References Cited UNITED STATES PATENTS 3,l8i,l254/1965 Vadopalas ..340/i73CR 3,636,526 1 1972 Feinleib 340/173 LS OTHERPUBLICATIONS Electronics-Sept. 28, 1970 pg. 61-72.

Primary Examiner-James W. Moffitt Attorney-Frank L. Neuhauser et al.

[57] ABSTRACT 10 Claims, 5 Drawing Figures V v t Q??? Pmimaw I 3.750.111

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I aussr A? GA i so I ATENI JUL 3 1 ms SHfEI 2 [If 2 ELECTRON BEAMADDRESSABLE ARCHIVAL MEMORY This invention relates to informationstorage devices and, in particular, to an archival memory utilizingelectron beam readout.

In the prior art, a wide variety of proposals for storing large amountsof information have met with varying degrees of success. However, asstorage capacities increase, the cost of the memory increases as well.Simi-- larly, the desire to reduce the size of the memoryhas increasedthe complexity of fabricating the memory further increasing its cost.

There is also a need in the art for archival" memories, i.e., long termpermanent storage memories in which information is written once andnever changed, but which can be read out frequently. In addition, theusual desiderata apply: high bit density, rapid access and low cost perbit.

Beam accessable memory systems appear to be an attractive approach forbuilding a large, high density memory with fast access time and low'cost. However, one form of this approach laser holographic storage inphotographic medium suffers from the disadvantage that the writing meansdiffers from the reading means and that complicated photographicdeveloping processes are. involved. This complicates the memory systemand prevents the addition of new information once the hologram has beenformed.

An electronbeam offers an alternative access mechanismfor the'memory.With an electron beam, to obtain high signal to noise ratio, a largereading beam current should be used. However, a large reading beamcurrent tends to produce a destructive rather than the desirednon-destructive readout.

Other storage targets for electron beam memory have been proposed andimplemented in the past. Among'them is thermoplastic recording.Thermoplastic recording, in which the information is stored as erasabledeformation of the material, in-general requires optical readout; i.e.,the writing and reading mechanisms are different.

In view of the foregoing, it is therefore an object of the presentinvention to provide an archival memory utilizing an electron beam forboth writing and reading.

It is a further object of the present invention to provide an archivalmemory utilizing a non-crystalline or an amorphous material exhibitingmetastable structural characteristics.

It is another object of the present invention to provide an archivalmemory utilizing a homogeneous, structureless target of non-crystallineor amorphous material in conjunction with electron beam deflectionapparatus. I

It is a further object of the present invention to provide an archivalmemory utilizing secondary electron emission effects for readout.

The foregoing objects are achieved in one embodiment of the presentinvention by providing a thin, e.g. less than 3,000A, film ofnon-crystalline material, e.g. vapor deposited amorphous germanium, as atarget for an electron beam. Also located adjacent the target, andoff-axis from the electron beam, is a secondary electron detector.

Writing is accomplished by irradiating selected bit sites of the targetto change the state of the material from amorphous to crystalline. Byutilizing a metastapoint of the material used for layer 12. Point A onthe ble material, the change from the amorphous to the crystalline stateis energetically very favorable while the change from crystalline toamorphous is impossible except under extreme conditions. Thus,information once stored is extremely difficult to erase; hence, thememory is archival. If desired, blank areas can be left on the target sothat additional information may be added to the memory as required.

In a second embodiment of the present invention, the target is coveredwith a metal layer, e.g. molybdenum.

A more complete understanding of the present invention may be obtainedby considering the following detailed description in conjunction withthe accompanying drawings, in which:

FIG. 1 illustrates one embodiment of the present invention.

FIG. 2 illustrates the relationship between volume and temperature forthe various states of the target material.

FIG. 3 illustrates an example of a ternary diagram giving thecomposition of suitable materials.

FIG. 4 illustrates an alternative form of the target in accordancewiththe present invention.

comprises a non-crystalline material target, an electron beam writingand reading apparatus and a secondary electron detector.

Specifically, target 10 comprises a high'thermal conductivity substratehavingdeposited thereon a layer 12 of non-crystalline material. Electronbeam generating and control apparatus comprises heater element 13,cathode l4 and deflection elements 15. These three elements serve togenerate and direct a beam of electrons 16 at target 10.

During an initial writing mode, the intensity of electron beam 16 issufficient to cause local heating at selected bit sites withinnon-crystalline layer 12 of target 10. The local heating serves tocrystalline portions of layer 12 forming crystalline areas such ascrystalline material 17.

During the reading operation, a beam of electrons of lower intensity isdirected at target 10 and the emitted secondary'electrons 18 aremonitored by the electron detector 19. Alternatively, the currentvariations due to secondary electron emission yield can be read out aschanges in voltage drop across output resistor 21 at output-terminal 22.

The overall operation of the embodiment illustrated in FIG. 1 may beunderstood by also considering FIG. 2 which illustrates an, importantparameter of noncrystalline and crystalline materials, the variation involume as a function of temperature for bulk glassy materials. Point Bon this curve represents the melting amorphous-liquid curve representsanoperating point of the material, that is, above this temperature thematerial comprising layer 12 is switched from the noncrystalline oramorphous state to the crystalline state. As can be seen by inspectionof FIG. 2, when the amorphous material is switched to the crystallinestate there is a decrease in the volume occupied by the material. Foramorphous germanium this decrease in volume amounts to approximately20-30 percent of the original volume.

Thus, as originally constructed, target comprises a substrate 11 havingdeposited thereon a uniform planar layer of non-crystalline material 12.The entire target is structureless, that is the storage site on thetarget are not defined by any physical features of the target. In aninitial writing operation a directed beam of electrons 16 is deflectedover the target to selected bit sites upon which it is desired to write.The impingement of electrons upon the selected bit sites locally raisesthe temperature of layer 12. At this point the material at the bit sitechanges from the amorphous to the crystalline state and undergoes achange in volume.

To read out the information stored in target 10, a lower beam exposure,as more fully explained below, is used. During the reading operation,the emission of secondary electrons 18 from the target is monitored bythe electron detector 19. Assuming the directed beam of electron 16impinges upon a storage site of decreased volume (crystalline), theyield of secondary electrons is reduced as compared to when the beam ofdirected electron l6 impinges upon an area of layer 12 in the amorphousstate. This variation in secondary electron yield as the directedelectron beam is impinged upon various storage sites in the targetprovides an output signal from detector 19 indicative of the informationstored in target 10. Alternatively, similar information can be obtainedby monitoring the current flowing through target 10 and readout resistor21, i.e., variations in secondary electron yield causes variations inthe current flowing through resistor 21.

As is well known, secondary electron emission" refers to the emission ofelectrons from a material bombarded by some form of primary" radiation.More specifically, it refers to the emission of electrons from a solidbombarded by higher energy electrons. At least two factors appear tocontribute to the change in the secondary electron yield from a switchedmemory site in accordance with the present invention. A first is thechange of the material into the crystalline state. This.

can produce a lower yield than the material in the amorphous state, asdiscussed by Chen, Norton and Wang, Applied Physics Letter 18, 443(1971). Other materials, such as antimony trisulphide, Sb S produce ahigher secondary emission yield in the crystalline state than in theamorphous state. Whether or not a given target produces a higher orlower yield is empirically determined. Insofar as the present inventionis concerned, it is immaterial whether the yield from the target area inthe crystalline state is higher or lower than that in the amorphousstate, so long as a variation is obtained.

A second factor in secondary emission is the depression itself, or,strictly speaking, the slope of the sides of the depression causing theelectron beam to strike the target at an angle other than normal to thetarget. This factor increases the secondary electron yield, as noted byA. J. Dekker in Solid State Physics, Vol. 6, Ed. by

Seitz and Turnbull, 1958, Academic Press, New York. In the secondembodiment of the present invention, with a metal overlayer, it is onlyrequired that the material deform, since the second embodiment relies onthis second factor alone.

FIG. 3 illustrates a ternary diagram as is frequently used in the art todesignate suitable material for useas layer 12 in accordance with thepresent invention. Memories in the prior art utilizing amorphousmaterials generally rely on theamorphous material as having bistablecharacteristics. That is, the amorphous material can be readily switchedbetween the amorphous and crystalline states. These suitable materialsare empirically found and generally designated as preferred areas suchas areas 31 and 32 in a ternary diagram.

The present invention, on the other hand, is concerned with materialswhose composition is defined by substantially the remainder of theternary diagram, in which the bulk materials are usually crystallinerather than glassy. A metastable, amorphous state is produced in thematerial by vapor depositing the material on a substrate cooled belowthe crystalline temperature. The resulting film is amorphous as vapordeposited but can be locally changed to the crystalline state by anelectron beam. These amorphous, vapor deposited films exhibit poorbistable characteristics since they transform irreversibly to thecrystalline state upon irradiation by an electron beam.

By utilizing a material for layer 12 that exhibits poor bistablecharacteristics, that is, a material which will switch irreversibly fromthe amorphous to the crystalline state, one can initially write on thetarget and subsequently read out the target a great many times withoutdestroying the information, as well as permanently store the informationtherein.

For example, the bulk glassy region within the ternary system of (Si,Ge), (Te, Se, 8,), (As, P, Sb) has been described by A. R. Hilton, C. E.Jones and M. Brau, Physics and Chemistry of Glasses, Vol. 7, Page I05(Aug. 1966). The compositions suitable for the archival memory of thepresent invention are generally those compositions which are crystallinein the bulk form and lie outside of thebulk glassy regions described byA. R. Hilton et al. The suitability of any composition depends onwhether vapor deposited thin film will be non-crystalline and on whetherthe films crystallization temperature can be attained by electron beamheating. For example, various Ge-Te thin films vapor deposited on cooledsubstrates are noncrystalline. The crystallization temperatures of thesefilms occurs from C to 500C depending on the richness of Ge content. Inprinciple, these films are all suitable for use in the presentinvention.

For the ternary system Ge-Te-Aa, films comprising Ge Te As and Ge 'leAs, have been found suitable for archival memory applications. These arebut some of the examples of compositions which are crytalline in bulkform but form vapor deposited noncrystalline thin films as describedabove. In general, for

greater than about 15 percent provides suitable films.

Other non-crystalline thin films can be formed by the vapor depositionof complex oxides with low substrate temperature. For example, E. K.Miller, B. .I. Nicholson and M. H. Francombe, Electrochemical Tech. Vol.1, page 158 (1963) shows that a grain by grain evaporation methodproduces non-crystalline glassy BaTiO thin films. Upon heating to above400C, the amorphous BaTiO thin film readily transforms irreversibly tothe denser crystalline form.

Similarly, the non-crystalline thin film may be a vapor-deposited,elemental semiconductor such as Ge on a cooled, maintained below 450C,substrate, The elemental amorphous semiconductor films also transformirreversibly to denser crystalline form upon heating to above itscrystallization temperature.

The film thickness of the non-crystalline material is preferably fromabout 500A to about I t. Again the thickness used depends upon the othermemory parameters. The desired characteristics of the substrate are itschemical inertness with the non-crystalline material and its highthermal conductivity characteristics. A 1,000A to l 11. thick molybdenumlayer on a silicon wafer is a suitable substrate. Other possibilitiesare tungsten on quartz, or a solid refractory metal such as molybdenumor tungsten.

Since the writing process is a thermal crystallization process, itinvolves complex interactions among the electron beam parameters, thematerial parameters, and the total thermal characteristics of thetarget. The goal is to achieve high density (fine beam diameter)electron beam induced crystallization in the noncrystalline thin film.For non-destructive read-out, the reading'electron beam energy shouldnot induce any change in the non-crystalline target. This can beachieved by lowering the beam voltage or the beam current.Alternatively, the beam dwell time at any spot on the target can bereduced to a sufficiently short time to prevent any heating to occur.

For example, writing has been accomplished in a target consisting of a1,650A film of Ge Te- As, amorphous semiconductor deposited upon 2,000Amolybdenum coated Si wafer. The molybdenum film is used to prevent anypossible chemical reaction between the Ge,,,Te As and the Si waferduring the vapor deposition process. The writing electron beamparameters were 4.5Kv, lOOnA, 1,000A in diameter and the beam was lineswept at a rate which cover 1,000A distance in SOnS (i.e. dwell time of50nS). The resultant electron beam recording in the target wasl,500-2,000A wide. Non-destructive electron beam reading wasaccomplished by the same electron beam parameters except the sweep ratecovered 1,000A in 5nS (i.e. dwell time of 5nS). Thus, reading ispossible with the same power electron beam but with decreased dwell timeto prevent any alternation of the target. Dwell time is defined as thetime required for the beam to travel its diameter.

In general, the specific values of the electron beam parameters, thephysical dimensions of the various layer of the thin film target and thedwell time depend on the material and thermal characteristics of thenoncrystalline thin film, the substrate and on each other.

In particular, the crystallization temperature is different fordifferent non-crystalline thin films. Thus, if all other parameters ofthe writing process are identical, the beam voltage, current and thedwell time vary with the crystallization temperature. Taking intoaccount the electronics, electron optics and material characteristicsrequirements, in general the writing process requires a beam voltage ofabout l.5l(v to 30Kv, and a beam current of approximately l0nA to 10 uA.The possible dwell time depends in part on the thermal characteristicsand in part on the crystallization kinetics but is approximately lnS tol0 #8.

While deflection system 15 is illustrated in FIG. 1 as comprising anelectrostatic deflection system, any suitable deflector may be utilized.FIG. 4 illustrates a pre ferred embodiment of the present inventionwherein deflector 15 is utilized as a coarse deflector and a matrixdeflection system 40 is used as a fine deflector, such as disclosed andclaimed byS. P. Newberry in US. Pat. No. 3,534,219. This deflectionsystem comprises a course deflector illustrated as two pairs oforthogonal electrostatic plates and a fine deflector composed of amatrix of lenslets forprecisely directing the electron beam overadjacent areas of a target. Selection of the lenslct and storage site tobe either written on or read out is made by an address command modulecontrolling the amplifiers coupled to the deflectors.

Specifically, a beam of electrons is deflected by coarse deflector 15 tothe matrix deflection system 40 which comprises sets of orthogonalconductors electrically connected to one another. That is, conductors 41which are parallel and extending in one direction are connected togetherand orthogonal conductor 42 are each connected one to the other. Coarsedeflector l5 deflects stream of electrons 16 to a particular aperture asdefined by orthogonal conductors 41 and 42. The potential applied toorthogonal conductors 41 and 42 then further directs electron beam 16 toa particular site on target 50 downstream therefrom. Target 50 comprisesa substrate 11 of a good thermal conductivity material having appliedthereover a layer of noncrystalline material 12 as previously described.In addition, target 50 has applied over non-crystalline layer 12 anadditional thin metal layer 43, for example, approximately A thick, suchas molybdenum. Any metal layer from 10 to 2,000A thick may be used asthe overlayer.

During the initial writing operation, electron beam 16 is directed atselected ,bitsites in storage target 50. As with the embodiment of FIG.1, electron beam 16 causes localized heating thereby changing thenoncrystalline material of layer 12 into the crystalline state asillustrated in H6. 4 by reference numeral 17. Metal layer 43 serves toseal target 50 so that should the material on layer 12 be volatizedduring writing it cannot escape from the surface of target 50, butrather remains trapped within target 50 as a minute crystalline mass atthe selected bit site.

Readout of target 50 is similar to that of target 10. However, theeffect relied upon to vary the yield of secondary electrons is differentfrom that of the embodiment illustrated in FIG. 1.

As previously noted, secondary electron emission yield is affected bythe geometric effect, that is, changes in the angle of incidence of theprimary electrons will change the yield of secondary electrons. Thus aselectron beam 16 traverses the previously irradiated bit site, the yieldof secondary electrons will show an appreciable increase as the beamtraverses the sloped portions of the sides of the bit site. Thesevariations in the secondary emission yield are detected by electrondetector 19. Alternatively, the effects of secondary emission can bedetected by monitoring the current flowing through target 50 and outputresistor 21. Variations in current due to variations in secondaryelectron emission yield indicate whether a non-crystalline orcrystalline storage site is being irradiated.

FIG. 5 illustrates a pair of waveforms 51 and 53 illustrating theoutputs obtained from targets 10 and 50 respectively. In reading outtarget 10, as the primary electron beam traverses a previously radiatedbit site in which thedensification and a phase change that has beeninduced reduces the yield of secondary electrons. Thus, the outputsignal from the electron detector 19 will contain a pronounced clip 52at the point representing the previously irradiated bit site. Incontrast to this, output waveform 53 contains a pair of peaks 54 and 55produced as primary electron beam 16 traverses the sloped side portionsof the densified bit site.

Having thus described the invention it will be apparent to those skilledin the art that various modifications may be made within the spirit andscope of the present invention. For example, as noted above, in thechoice of material for amorphous layer 12, compounds other thangermanium, tellurium and arsenic may be utilized. What we claim as newand desire to secure by Letters Patent of the United States is:

1. An archival memory system comprising target means comprising a thinfilm of metastable, non-crystalline material overlying a high thermalconductivity substrate; electron beam deflection means for initiallywriting on said target means by electron beam heating selected bit siteson said target to change the material at said sites from thenon-crystalline state at one volume to the crystalline state at anothervolume; said electron beam deflection means also reading said target byirradiating bit sites therein; and secondary electron emission detectionmeans for sensing variations in secondary electron emission yield duringsubsequent reading operations as said deflection means irradiates bitsites in said target means. 2. An archival memory as set forth in claim1 wherein said electron beam deflection means comprises:

electron beam generating means; a first deflection means; and a seconddeflection means comprising a planar array of a plurality of deflectionelements arranged in a matrix, wherein the electron beam from saidgenerating means is deflected by said first deflection means to one ofthe deflection elements of said second deflection means which, in turn,directs said electron beam to said target means.

3. An archival memory system as set forth in claim 1 wherein saidmaterial comprises a complex oxide vaper-deposited upon a cooledsubstrate.

4. An archival memorysystem as set forth in claim 1 wherein saidmaterial comprises amorphous germanium.

5. An archival memory system as set forth in claim 4 wherein saidsubstrate comprises a molybdenum layer overlying a silicon wafer.

6. An archival memory as set forth in claim 1 wherein said targetcomprises a molybdenum layer overlying said amorphous material.

7. An archival memory as set forth in claim 6 wherein said materialcomprises a 3 ,000A thick layer composed of germanium, arsenic andtellurium overlying a substrate and having a A layer of molybdenumthereover.

8. An archival memory as set forth in claim 7 wherein said materialcomprises at least 15 percent germanium.

9. An archival memory as set forth in claim 8 wherein said materialcomprises a vapor deposited layer having the composition Ge Te As 10. Anarchival memory as set forth in claim 8 wherein said material comprisesa vapor deposited layer having the composition Ge Te As I i i

1. An archival memory system comprising target means comprising a thinfilm of metastable, noncrystalline material overlying a high thermalconductivity substrate; electron beam deflection means for initiallywriting on said target means by electron beam heating selected bit siteson said target to change the material at said sites from thenoncrystalline state at one volume to the crystalline state at anothervolume; said electron beam deflection means also reading said target byirradiating bit sites therein; and secondary electron emission detectionmeans for sensing variations in secondary electron emission yield duringsubsequent reading operations as said deflection means irradiates bitsites in said target means.
 2. An archival memory as set forth in claim1 wherein said electron beam deflection means comprises: electron beamgenerating means; a first deflection means; and a second deflectionmeans comprising a planar array of a plurality of deflection elementsarranged in a matrix, wherein the electron beam from said generatingmeans is deflected by said first deflection means to one of thedeflection elements of said second deflection means which, in turn,directs said electron beam to said target means.
 3. An archival memorysystem as set forth in claim 1 wherein said material comprises a complexoxide vapor-deposited upon a cooled substrate.
 4. An archival memorysystem as set forth in claim 1 wherein said material comprises amorphousgermanium.
 5. An archival memory system as set forth in claim 4 whereinsaid substrate comprises a molybdenum layer overlying a silicon wafer.6. An archival memory as set forth in claim 1 wherein said targetcomprises a molybdenum layer overlying said amorphous material.
 7. Anarchival memory as set forth in claim 6 wherein said material comprisesa 3,000A thick layer composed of germanium, arsenic and telluriumoverlying a substrate and having a 100A layer of molybdenum thereover.8. An archival memory as set forth in claim 7 wherein said materialcomprises at least 15 percent germanium.
 9. An archival memory as setforth in claim 8 wherein said material comprises a vapor deposited layerhaving the composition Ge39.4Te60.3As0.3.
 10. An archival memory as setforth in claim 8 wherein said material comprises a vapor deposited layerhaving the composition Ge26Te70As4.